Ultrasonic inspection systems for detecting various characteristics of articles moving before transducers are well-known in the art. These systems have been utilized for a number of applications. For instance, the systems can be utilized to detect the number of targets moving past transducers. Further, such ultrasonic detection systems have been utilized to detect the rate of movement of targets moving within a range of such ultrasonic detectors.
An example of an ultrasonic inspection system utilizing known techniques is illustrated by U.S. Pat. No. 3,321,959 which utilizes the Doppler shift for inspection purposes. A system of the Doppler type typically utilizes a transmitter and a receiver which are separate and discrete elements.
Another sonic system is illustrated in U.S. Pat. No. 3,424,507. This system also illustrates the prior art need to utilize both a transmitter and a separate receiver for locating objects. This utilization of separate transmitting transducers and receiving transducers is typical of the prior art and the inability of prior art systems to effectuate detection without the use of separate and discrete transducers as receivers and transmitters. A further example of this prior art is illustrated in U.S. Pat. No. 3,504,532 which illustrates a non-destructive testing system. Again, the system uses distinct and separate transmitting and receiving transducers.
These prior art systems have "built-in" disadvantages. First, the transmitting transducer and the receiving transducer are separate elements. There is an inherent need in such a system for tuning of these devices. For example, in the event of frequency drift of the transmitting transducer, there must be a corresponding adjustment made in the receiving transducer in order to insure accurate detection by the system.
Naturally, there can be no frequency tracking in such a system which is accurate. In other words, any drift in the frequency on the part of the transmitting transducer is of necessity followed at a discrete point in time later by an appropriate adjustment in the frequency response of the receiving transducer. This time lag, of course, results in inaccuracies, limitations, failure and other inaccuracies in such a system.